The present invention relates to a high frequency power amplifier and a technology effective for application to a wireless communication apparatus such as a cellular phone or the like with the high frequency power amplifier built therein, and particularly to a technology for speeding up an open-loop type high frequency power amplifier capable of controlling a power supply voltage for output power FETs constituting the high frequency power amplifier to thereby control output power and improving efficiency thereof at a low output.
An output section on the transmitting side of a wireless communication apparatus (mobile communication apparatus) such as a mobile phone, a cellular phone or the like has built therein a module (called an xe2x80x9cRF power modulexe2x80x9d) in which a high frequency power amplifier (generally provided in a multistage configuration) using semiconductor amplifying elements such as MOSFETs (field effect transistors), a GaAs-MESFET, etc. and its bias circuit are integrated into one.
Meanwhile, a cellular phone generally has a system configured in such a manner as to change an output (transmission power) so as to adapt to an ambient environment according to power level designation information sent from a base station in accordance with a use environment to thereby start a call and in such a way as not to cause interference between the cellular phone and other cellular phones.
An RF power module of a transmission-side output stage of a portable phone of a cellular type or system such as a North American Standard System of a 900 MHz band, an European GSM (Global System for Mobile Communication) system or the like is provided with an APC (Automatic Power Control) circuit which effects feedback on a gate bias circuit for generating a gate bias voltage of each output power element so as to detect a DC level of an output and provide output power necessary for a call (e.g., Unexamined Patent Publication No. 2000-151310). Such a control system is generally called a xe2x80x9cclosed-loop type or systemxe2x80x9d.
However, the system for controlling the output power by the APC circuit is accompanied by a problem that a circuit scale becomes large and a packaging density is decreased. Therefore, there is provided a system for controlling a power supply voltage for each output power FET based on an output level designation signal corresponding to a required output level to thereby cause a signal having a level corresponding to the required output level to be outputted from a high frequency power amplifier. This system is called an xe2x80x9copen-loop systemxe2x80x9d and has the advantage of being capable of reducing a circuit scale as compared with the close-loop system.
Meanwhile, the conventional high frequency power amplifier of open-loop system has a problem in that even if linearity of its output is good, the efficiency at a low output is poor. It was found that a problem also arose in that the speed of response to the output level designation signal was slow. It has been cleared that when a power supply voltage for each output power FET is changed based on amplitude information about a transmit signal to thereby effect amplitude control particularly in a high frequency power amplifier employed in a communication apparatus in which voice-signal communications are performed according to a GMSK (Gaussian filtered Minimum Shift Keying) modulation system for phase-shifting the phase of a carrier wave according to transmit data, and data communications are performed according to an EDGE (Enhanced Data Rates for GMS Evolution) modulation system in which an amplitude shift is further added to a phase shift in GMSK modulation, the speed of response in the circuit is not sufficient.
An object of the present invention is to provide a high frequency power amplifier capable of performing open-loop type transmission, which is satisfactory in output linearity and excellent in efficiency at a low output.
Another object of the present invention is to provide a high frequency power amplifier excellent in response to a control signal where a power supply voltage for each output power FET is controlled to control an output level.
A further object of the present invention is to provide a multifunction type high frequency power amplifier capable of performing transmission based on an EDGE modulation system for changing a power supply voltage for each output power FET based on amplitude information about a transmit signal to thereby effect amplitude control in addition to a GMSK modulation system for phase-shifting the phase of a carrier wave according to transmit data.
A still further object of the present invention is to provide a high frequency power amplifier which makes it possible to increase a call time of a wireless communication apparatus and the life of a battery.
The above, other objects and novel features of the present invention will become apparent from the description of the present specification and the accompanying drawings.
Summaries of typical ones of the inventions disclosed in the present application will be explained in brief as follows:
An open-loop type high frequency power amplifier wherein a power supply voltage for each output power FET is controlled based on a signal for specifying or designating an output level to thereby output a signal having a level corresponding to the required output level, is provided with a bias voltage generating circuit which generates a gate bias voltage for each output power FET according to a voltage outputted from a power control circuit for controlling the power supply voltage for each output power FET based on the signal for designating the output level.
According to the above means, a high frequency power amplifier can be obtained which is capable of improving efficiency at a low output while output""s linearity is being ensured. Thus, since the efficiency at a low output relatively high in the frequency of its use is enhanced, the total power consumption is reduced, and a wireless communication apparatus such as a cellular phone or the like using this type of high frequency power amplifier is capable of increasing a call time and the life of a battery used therefor.
Preferably, the power control circuit, which controls the power supply voltage for each output power FET based on the output level designation signal, makes use of a circuit which comprises an operational amplifier circuit for amplifying the output level designation signal and a MOSFET whose gate is controlled based on the output of the operational amplifier circuit to thereby output a power supply voltage for the output power FETs from a drain thereof and which feeds back the drain voltage of the MOSFET to the operational amplifier circuit to thereby generate a desired power supply voltage. There is further provided a buffer circuit having a bipolar transistor as an active element between the operational amplifier circuit and a gate terminal of the MOSFET which outputs the power supply voltage for the output power FETs. Consequently, a high frequency power amplifier is obtained which improves response to a control signal and is also capable of performing transmission based on an EDGE modulation system for changing a power supply voltage for each output power FET based on amplitude information about a transmit signal to thereby effect amplitude control.
Further, preferably, a phase compensating circuit comprising a CR circuit or the like in which a capacitor and a resistor are connected in parallel, is provided between the operational amplifier circuit and the gate terminal of the MOSFET for outputting the power supply voltage for each output power FET. Thus, a phase allowance is developed in the power control circuit and hence distortion of an output can be reduced.
Bias voltage switching means is provided which makes it possible to supply a bias voltage corresponding to a required output level to the high frequency power amplifier as an alternative to the bias voltage outputted from the bias voltage generating circuit. Thus, when a signal phase-modulated by the EDGE modulation system is inputted to the high frequency power amplifier and an output""s amplitude control information signal is inputted to the power control circuit as an alternative to the signal for designating the output level, the bias voltage corresponding to the required output level is supplied to the high frequency power amplifier as an alternative to the bias voltage outputted from the bias voltage generating circuit, whereby a system capable of performing transmission based on the EDGE modulation system can be configured, and a bias voltage corresponding to an output level is supplied to the high frequency power amplifier upon such transmission to thereby allow the operation of the high frequency power amplifier.